1. Field of the Invention
The present invention relates to a semiconductor laser light source tunable in wavelength (hereinafter, a semiconductor laser may be simply referred to LD) for obtaining a specific reference wavelength which is used in the field of optical communication, in particular, for WDM (Wavelength Division Multiplex) transmission or the like, and relates to a tunable LD laser light source using a fiber grating as a wavelength selecting means.
2. Description of the Earlier Development
It is known that in a Fabry-Perot type of resonator which has one facet of an LD, having an antireflection coating through which a wavelength selected by using an element having a wavelength selection property is fed back into the LD and which has the other facet for forming an external resonator together with the one facet, a laser oscillation is carried out when the gain condition and the phase condition overcome a loss such as a scattering loss and the like.
As described above, a tunable LD light source is formed by selecting a laser beam having a desired wavelength from the output beam of the laser-oscillating LD, and thereafter by feeding it back into the LD.
Recently, such a tunable LD light source comes to be an indispensable one in the WDM transmission system which attracts public attention as a large capacity transmission technology in an optical communication network, which is required with tendency to use multimedia.
An earlier LD light source tunable in wavelength will be explained with reference to FIG. 5, as follows.
FIG. 5 shows an example of construction of an earlier tunable LD light source.
In the earlier example shown in FIG. 5, one of the facets of the LD 10 has a cloven surface 11 and the other of the facets has an antireflection coating 12 provided thereon.
The light beam outputting through the side of the antireflection coating 12 of the LD 10 is changed to a parallel beam through a lens 31. In front of the lens 31, a total reflection mirror 50 is arranged through a tunable bandpass filter 40. The total reflection mirror 50 and the cloven surface 11 of the LD, which has a low reflection coating thereon form an external resonator.
In front of the cloven surface 11 of the LD 10, a lens 33 is arranged. The laser beam transmitted through the lens 33 is transmitted through an optical fiber 60 and is output out of an output port 11.
Next, the operation of the tunable LD light source shown in FIG. 5 will be explained, as follows.
The light beam emitted from the side of the antireflection coating 12 of the LD 10 is changed to a parallel beam through the lens 31, and then is directed to a tunable bandpass filter 40 to transmit only a light beam having a specific wavelength. Thereafter, the light beam is reflected by the total reflection mirror 50 to change the advancing direction thereof 180.degree. and passes through the tunable band pass filter 40 and the lens 31 again, and directs back to (is fed back to) the LD 10. The light beam directed to the LD 10 is reflected by the cloven surface 11 having a reflectance of several tens of % and is returned to the LD 10 again.
A laser oscillation is carried out in the external resonator which is constituted by the cloven surface 11 and the total reflection mirror 50. The tunable band pass filter 40 of very narrow bandwidth, i.e., having a bandwidth (FWHM, i.e., Full width half maximum) of the transmission range of about 0.5 nm, is used.
Thus, the light beam obtained by laser oscillation is coupled to the optical fiber 60 through the lens 33 to enter the output port 61. In such a construction, a tunable LD light source can be made by making the transmission wavelength of the tunable band pass filter 40 changeable.
However, because such an earlier LD light source tunable in wavelength shown in FIG. 5 used only a tunable bandpass filter 40 as a wavelength selection means and required a narrow bandwidth (FWHM) of the transmission range of the filter. As a result, the cost for the tunable band pass filter 40 was very high and about 0.5 nm was the upper limit of the bandwidth (FWHM) of the transmission range of the tunable band pass filter 40. According to an earlier tunable LD light source, it was difficult to obtain a laser oscillation having a narrow spectral width.
Because high-precision control for the tilt angle of the tunable band pass filter 40 to the incident light beam and for the temperature thereof were required in order to enhance the repeatability of selected wavelength, such an earlier tunable LD light source had the disadvantages in a productional aspect in that a complicated structure, a high optical axis adjustment technique and a high mounting technique were required.